Re: Do we still care about compilers without __seg_fs and __seg_gs support??

[Uros Bizjak]

On Sat, Dec 20, 2025 at 12:24 AM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> As of Linux 6.16, we require:
>
>         gcc 8.1 or higher
>         clang 15.0.0 or higher
>
> If my reading of the release notes is correct, then both versions *should*
> supported __seg_fs and __seg_gs, but we have:
>
> config CC_HAS_NAMED_AS
>         def_bool $(success,echo 'int __seg_fs fs; int __seg_gs gs;' | $(CC) -x
> c - -S -o /dev/null)
>         depends on CC_IS_GCC
>
> We don't even try on clang.
>
> Being able to actually rely on the compiler for this would make a lot of
> things cleaner.  For one thing, I'm trying to untangle a bunch of ugliness in
> the code sharing between realmode and proper flat mode code...
>
> Uros, you seem to have touched this code as recently as earlier this year; any
> thoughts?

To enable __seg_gs support with clang, you will first need the patch at [1].

Second, clang does not emit %gs: prefix by itself when the address
from __seg_gs named address space is used in asm(). This can be worked
around by leaving  __percpu_arg(x) defined to __percpu_prefix "%" #x
for clang.

Finally, you will hit issue #93449.

[1] https://lore.kernel.org/lkml/20240526175655.227798-1-ubizjak@xxxxxxxxx/

Uros.